Lithium rechargeable batteries have recently drawn attention as power sources for small, portable electronic devices. Lithium rechargeable batteries use organic electrolytic solutions and have discharge voltages twice as great as conventional batteries using alkaline aqueous solutions, and therefore have high energy density.
A rechargeable battery can have various shapes, for example the battery can be cylindrical, prismatic, or the like. The rechargeable battery is generally fabricated by housing an electrode assembly (in which a separator is positioned between bar-shaped positive and negative electrodes) in a case and mounting a cap assembly having an outer terminal on the case. The positive and negative electrodes include coated regions and uncoated regions. The coated regions are formed by coating a current collector with an active material. A conductive tab is mounted on the uncoated region. The conductive tab collects current generated from the positive and negative electrodes. The conductive tab is fixed to the electrode assembly by welding or the like, and collects and guides the current generated from the positive and negative electrodes to positive and negative outer terminals.
Positive and negative electrodes of the lithium rechargeable battery are fabricated by coating an active material composition on a current collector. The active material composition includes an active material and a binder for attaching the active material to the current collector. The binder is included to promote adhesion and attachment of the active material to the current collector.
The adhesion and attachment of the active material to the current collector by the binder critically influences long-term cycle-life characteristics of lithium rechargeable batteries. When the binder has low adhesion, the active material can detach from the current collector during long term use of the battery. Accordingly, adhesion of the active material to the current collector relies on the binder providing adhesion and attachment of the active material to the current collector.
The binder may be a polymer binder, for example a fluorinated resin such as polyvinylidene fluoride, polytetrafluoroethylene, and the like. Alternatively, the binder may be a styrene-butadiene rubber or the like. Of these binders, polyvinylidene fluoride is generally known to impart stronger adhesion of the active material, and is therefore widely used.
However, when only polyvinylidene fluoride is used as the binder, lithium in the active material undergoes a non-reversible reaction with the fluorine group in the binder. This reaction produces LiF upon charging and discharging of the lithium rechargeable battery, thereby reducing the amount of lithium available for charge-discharge of the lithium rechargeable battery and decreasing battery efficiency. Accordingly, there is a need for a binder that can improve adhesion of the active material to the current collector.
The information disclosed in this Background section is presented to further the understanding of the present invention. Applicant does not admit that all information disclosed in this Background section is prior art to the present invention.